If it's pouring back out the fill hole, it's probably full. How much are you trying to put in it? They don't really hold much.

 

Harbor freight sells a nice little fluid pump for under $5. I used one when I replaced the seals and gaskets on my front differential the gasket on my rear axle. Also good for brake fluid on a motorhome. The seals on the pump tend to die quickly, but the little pumps are so cheap they're disposable. Get some nitrile gloves, too

 

if it's pouring out of the upper fill opening (which is to the right of and about an inch above the drain plug) then the T-case is full.